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VE2HAR > MT63 13.03.05 19:04l 160 Lines 6157 Bytes #-7304 (0) @ WW
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Subj: Re: [MT63] Re: Crest Factors
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Date: Sun, 13 Mar 2005 10:27:59 -0700
Subject: Re: [MT63] Re: Crest Factors
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I think that we need not worry so much what is calculated as why we do
this.
Because all real amplifiers have a limit in the amplitude of the signal
that they can pass, it is important to insure the signal's amplitude
never exceed this limit. Personally, I set my peak output power about 1
to 2 DB lower that what I can produce with a pure CW signal. The safest
way to accomplish this is using an oscilloscope and observe the
transmitter output. We old-timers remember in the 60's that this was
mostly done to insure linearity of a single sideband signal. Maybe we
should go back to this technology? Look for the signal on the scope CRT
display to be at least 1/2 graticule less than what you were getting
with the CW signal.
The problem with using analog meters is that the meter movement can not
keep up with the dynamics of the signal, unless we are trying to measure
a pure carrier. With more than one carrier within the transmitted
signal, the amplitude peak of the signal complexes. The more
(non-harmonically related) carriers, the more complex the waveform becomes.
Not to confuse the issue, but my watt meter is a thermal sensor unit. It
uses RF power to heat a small element via a load/thermistor. The
frequency range is 1-1000 MHz, though in actuality it doesn't matter,
since the measurements I made were at the same frequency (14.110 MHz).
My transmitter is capable of producing 105 watts pure CW carrier.
Observing on a scope, there is no modulation, or compression of the
signal. Now comes MT63. Trying to obtain the same output was not
possible as there is a noticeable gain compression beginning at 85
watts, so to get an honest reading without introducing gain compression
in the generator/amplifier chain, I reduced my measurements to 80 watts
for the reference carrier. On order to transmit the same peak RF signal
that the CW carrier produces with 80 watts, I find that I can only
produce 13.5 watts on the thermal watt meter using MT63 and idle. I saw
no noticeable change from idle to sending a file.
Since all of us interested in MT63 or other modes using multiple-tone
digital modes are also conscious of our resource neighbors, or should
be, then I suggest we use a scope wherever/whenever possible during
set-up to ensure a clean signal. (I am assuming this is the reason that
this thread began in the first place)
I know there are several out there who think that they must squeeze
every last drop out of the transmitter. Remember, distortion does not
contribute to readability, and actually degrades readability and even
trippling the power out is just noticable by the receiver (an 's' unit).
so, my theory is that running with my analog meter showing about 10 - 12
watts means that I have my transmitter optimized, and should be sharing
with my peers on neighboring modes/frequency without causing problems to
their use.
Steve/WM5Z
Tomi Manninen wrote:
> On Sun, 2005-03-13 at 17:35, Mark Miller wrote:
>
> > Envelope crest factor
> > To determine the envelope crest factor, first determine the power of a
> > sinusoidal signal that has the same peak voltage as the modulated
> signal's
> > peak voltage. This value is then divided by the average power of the
> > modulated signal. The result is defined as the translated signal's
> envelope
> > crest factor.
>
> So envelope crest factor is peak crest factor minus 3 dB, right?
>
> Anyway, a while back I did some calculations with the numerical
> sample data right out of the MT63 modem. The (peak) crest factor
> number I got was around 13 dB, slightly dependent on the data.
>
> --
> Tomi Manninen / OH2BNS / KP20JF74
>
>
>
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